The NTSC television system is used in many countries, such as the United States, Japan, etc. Furthermore, a number of wide-aspect high definition television broadcasting systems are currently planned to start. In these systems a high definition television (hereinafter referred to as HDTV) broadcasting system and a second generation EDTV (Extended Definition Television) broadcasting system have aspect-ratios wider than that of the existing NTSC television receivers. However they are different from each other.
To comply with the Hi-Vision system broadcasting (16:9 aspect-ratio, 1125 scanning lines) which is a type of the HDTV broadcasting system carried out in Japan, a simplified type of wide aspect screen television receivers equipping a MUSE-NTSC converter inside or outside thereof for performing the sequential scanning of 525 scanning lines at the 16:9 aspect-ratio has appeared. Moreover, there exists a second generation EDTV broadcasting which is also available to receive such a wide aspect-ratio broadcasting signal by implementing the sequential scanning of 525 scanning lines at the 16:9 aspect-ratio.
Among the wide aspect-ratio television receivers which equip a display device, e.g., a cathode ray tube (hereinafter referred to as CRT) with the 16:9 aspect-ratio and are capable of sequential scanning 525 scanning lines; a television receiver which is capable of displaying a picture on a screen of the 16:9 aspect-ratio by switching between a normal mode for fully scanning the 525 scanning lines and a zoom mode for reduced scanning lines, i.e., 3/4 (about 75%) of 525 scanning lines by expanding the vertical picture amplitude; has been developed.
FIG. 1(a) shows the normal mode in which 525 scanning lines are displayed on a CRT with the 16:9 aspect-ratio, while FIG. 1(b) shows the zoom mode in which 525.times.(3/4) scanning lines ape displayed on the same CRT.
On such a wide aspect-ratio television receiver which is capable of switching between the normal mode and the zoom mode, it is considered to display a sub-picture in the PIP (Picture In Picture) display system in which horizontal and vertical direction thinning-out ratios are equal (for example, both horizontal and vertical direction thinning-out ratios are 1/3). This PIP technique has been adopted on a television receiver equipped with a 4:3 aspect-ratio CRT. FIG. 2 shows the PIP display mode on an television receiver equipped with a conventional 4:3 aspect-ratio CRT. FIG. 2 further shows the case in which the sub-picture horizontal sampling frequency fs is taken as fs=910 fH (where fH is horizontal scanning frequency of the main-picture), and the horizontal picture amplitude of the sub-picture is taken as (910 fH).times.(1/3) and its vertical picture amplitude as 525.times.(1/3).
Here suppose a case where the output of an MUSE-NTSC converter which converts MUSE signals into the NTSC signals is used as input signals for the PIP mode sub-picture. Then, following two modes (1) and (2) are considered as the output of the MUSE-NTSC converter,
1) A first specific mode in which it is possible to display a picture with 100% circularity when 525 scanning lines are displayed on a television receiver with a 16:9 aspect-ratio CRT. This mode is called the squeeze mode and is shown in FIG. 3(a). PA1 2) A second specific mode in which it is possible to display a picture with 100% circularity, having a blank space of 1/4 (about 15%) of 525 scanning lines at the upper and the bottom sections when 525 scanning lines are displayed on a television receiver equipped with a 4:3 aspect-ratio CRT. This mode is called as a letter-box mode and is shown in FIG. 3(b).
When the normal mode or the zoom mode is used for the displaying a main-picture, while the squeeze mode or the letter-box mode is used for displaying a sub-picture, the following four patterns as shown in FIGS. 4(a) through 4(d) can be considered.
FIG. 4(a) shows a case where the main-picture is displayed in the normal mode and the sub-picture is displayed in the squeeze mode. FIG. 4(b) shows a case where the main-picture is displayed in the zoom mode and the sub-picture is displayed in the squeeze mode, FIG. 4(c) shows a case where the main-picture is displayed in the normal mode and the sub-picture is displayed in the letter-box mode, and FIG. 4(d) shows a case where the main-picture is displayed in the zoom mode and the sub-picture is displayed in the letter-box mode.
In any combination of these modes for displaying the main-picture and the sub-picture, even if it is tried to superimpose the sub-picture on the main-picture, both of them are in the same 16:9 aspect-ratio, the same 100% circularity and the same size, it will be impossible on a conventional PIP mode with the equal horizontal and vertical direction thinning-out ratios. That is, the sub-picture of a circle as shown in FIG. 4(a) will have its shape deformed into a circuilarity corresponding to the 16:9 aspect-ratio, like a vertically long ellipse of the 4:3 aspect-ratio as shown in FIG. 4(b), a horizontally long ellipse of the 16:9 aspect-ratio as shown in FIG. 4(c), and a circle of the 4:3 aspect-ratio and the 100% circularity in FIG. 4(d). Even in the same main-picture mode, a sub-picture in different aspect-ratio, circularity and size is displayed as shown in FIGS. 4(a) and 4((c), and FIGS. 4(b) and 74d) depending upon a sub-picture mode. Consequently the result will be an unsightly sub-picture.
As described above, in a conventional PIP mode of the equal horizontal and vertical direction thinning-out ratios, there is the problem that in any combination of the main-picture mode and the sub-picture mode, it is impossible to display a sub-picture in the same size as the main-picture with the 16:9 aspect-ratio and 100% circularity, so that there is the drawback of unsightliness.